Metabolic Pathways in Monocyte Development Identified Using HoxB8 Model Cells

Metabolism is a hot topic in immunology. Immune cells carry out diverse functions by exploiting different pathways of taking in nutrients and generating energy. Accordingly, metabolic processes such as glycolysis and oxidative phosphorylation are now front and center in understanding how cells behave in the context of normal and pathogenic immune responses.

JBC Young Investigator Pui Lee, MD, PhD, an attending in pediatric rheumatology at Boston Children’s Hospital, took advantage of a technology available through the Joint Biology Consortium to better understand the role of metabolism in monocytes. Circulating monocytes come in two “flavors”: 1) classical monocytes that are recruited into inflamed tissues to become active macrophages and 2) patrolling monocytes that crawl along the luminal walls of blood vessels looking for evidence of injury or infection. To study the relationship between these types of monocytes, Dr. Lee needed a system in which he could watch these cell types develop. Even in the mouse this is a difficult task, because monocyte precursor cells are rare and impossible to isolate from bone marrow without extensive manipulation.

To address the problem, Dr. Lee used an innovated modeling system developed originally at the University of California San Diego. There, investigators had noted that marrow cells induced to express the myeloid transcription factor HoxB8 become stable myeloid cell progenitors that can be maintained and expanded in culture. When expression of this gene is shut off, these cells then pick up where they left off, differentiating into normal monocytes, neutrophils, and other myeloid lineages. Accordingly, reversible HoxB8 expression provides a new tool to understanding the biology of otherwise hard-to-culture myeloid cells.

Dr. Lee developed a HoxB8-based system in which he could use two fluorescent markers expressed by developing monocytes to easily track their development in vitro. As suggested by published studies, he confirmed that patrolling monocytes arise from classical monocytes. Using a small molecule screen – a library of chemical “poisons” for specific cellular pathways – he then sought to identify novel pathways in this process. In this way, Lee and his team established a previously unrecognized role for the metabolic integrator mTOR in monocyte development, which he then confirmed in living mice bearing constitutive or inducible mutations in related genes. This work, published in Science Immunology, has formed the basis of Dr. Lee’s pending K08 application to understand the factors that activate mTOR and the impact of metabolism on the role of monocytes in inflammatory diseases.1

The HoxB8 system can be used to study murine myeloid cells of all types, with protocols available through the JBC not only for generating monocytes but also neutrophils, dendritic cells, mast cells, and osteoclasts (see Figure). Since these myeloid precursor cells can be expanded indefinitely, they also can then be manipulated genetically, using gene-editing technology such as CRISPR. Multiple JBC members are now using this tool, with the help of the JBC HoxB8 Core to overcome technical hurdles and thereby enable exploration of new hypotheses in rheumatology and immunology. For more information see the http://www.jbcwebportal.org/cellular-systems-core/technologies/hoxb8.